1. Technical Field
The present invention relates to a fuel cell unit in the form of an extruded monolith comprising channels of which every second constitutes a fuel channel and every other second an oxygen channel. The invention also relates to a fuel cell device comprising (including) several such fuel cell units. In particular, the invention is applied in connection with packing of fuel cell units to a larger system of fuel cell units; for example, a fuel cell device to be used as a power source in a vehicle.
2. Background Art
In the pursuit of environmentally enhanced power sources, for example with respect to propulsion systems for vehicles, fuel cells have been the subject to extensive research. There are many types of fuel cells that use different types of fuel for different purposes. The production of electricity, however, is a common feature of substantially all fuel cells.
All fuel cells are constructed in a layer structure comprising a fuel side, an oxygen side, a membrane and two electrically conducting plates in the form of an anode and a cathode. The membrane is an electrical insulator at the same time as it works as an electrolyte that admits ionic conduction between the anode and the cathode, which are placed on each side of the membrane. The fuel side is normally placed at the anode side and the oxygen side is normally placed at the cathode side. For most fuel cells, the anode, the cathode, and the electrodes consist of a porous carbon material that is coated with a catalyst material such as platinum (Pt). The catalyst material catalyses a reduction of the fuel at the anode side by means of a reduction of electrons and catalyses an oxidation of the oxygen at the cathode side by means of a supply of electrons. These two reactions cause an electron migration, that is, an electrical current from the anode side to the cathode side via an electrode connection. The ionized particles from either the anode side or the cathode side diffuses through the membrane and reacts on the opposing side by forming some kind of compound, for example water. If there is hydrogen on the fuel side and the membrane allows ionized hydrogen to diffuse, the process and arrangement may be referred to as “Proton Exchange Membranes” (PEM). If the membrane allows ionized oxygen to diffuse from the cathode side, reference may be made to “Oxide Fuel Cells” (OFC).
In a structure of a type referred to as “Solid Oxide Fuel Cell” (SOFC), a ceramic solid phase membrane (electrolyte) is utilized. A suitable material that is used is “dense yttrium stabilized zirconium dioxide,” which is an excellent conductor for negatively charged oxygen ions at high temperatures around 1830 degrees F. (1000 degrees C.). At such a temperature, it is possible to have an inner reforming of carboniferous fuels.
When using a fuel cell, it is necessary to consider a number of parameters such as weight, volume, degree of efficiency, working temperature, material, fuel, exhausts and the like depending upon within which field of usage the fuel cell shall be used.
In order to satisfy the power need of a larger unit such as a vehicle, more fuel cells are needed. A way to solve the problem with the mounting of the many fuel cells is to extrude a fuel cell unit in the form of a monolith with a substantially honeycomb structure comprising (that includes) a number of fuel cells which thus form a larger, more compact fuel cell. In some cases, a single monolith with a honeycomb structure will not be able to be made large enough to supply sufficient electricity to power such a larger device as an automobile or other type of vehicle due to manufacturing reasons. This can mean that a mounting of several fuel cell units in the form of monoliths with a honeycomb structure is therefore made necessary.
It is previously known to extrude an SFOC fuel cell unit in the form of a monolith with a honeycomb structure in a material of yttrium stabilized zirconium dioxide which constitutes a membrane that conducts ions, but is not electrically conductive. The fuel cell unit then consists of square/rectangular channels defined by extruded walls of yttrium stabilized zirconium dioxide which form rows of fuel conduit channels with a square/rectangular cross-section with a pole of a conducting catalyzing material on the inside of the channel, and rows of oxygen conduit channels with a square/rectangular cross-section with a pole of a conducting catalyzing material on the inside of the channel. The rows of channels are placed in such a way that every second row is a fuel conduit channel and every other second row is an oxygen conduit channel. The fuel conduit channels and the oxygen conduit channels are of equal lengths and sizes, why every short side of the monolithic fuel cell unit is covered by a covering plate with a system of channels that is designed to conduct the fuel and the oxygen, respectively, to the correct row, i.e. to the correct channel. The monolithic fuel cell unit may be connected to other similar fuel cell units, thus acquiring a compact system of fuel cell units with desired power, by designing a larger covering plate to cover the short side of the system of monolithic fuel cell units that have been connected and where the covering plate has been equipped with a system of channels which supplies the fuel channels and the oxygen channels with the correct fluid; respectively, the fuel and oxygen.
Even if previously known systems function well, enhancements may be made concerning acquiring a more compact system of fuel cell units (fuel cell device). According to previously known technology, the covering lid that covers the short side of the fuel cell device is designed with specially adapted channels that shall fit the fuel, oxygen and exhaust channels. For the fuel cell device to function properly, high demands are made upon the fitting and tightness between the covering plate with its channels and the shaped rows of channels in the fuel cell units. The manufacture of such a plate may be expensive, and the special demands make the device quite inflexible. Even if separate bottom-plates were used for the separate extruded fuel cell units, an adaptation should be necessary for the connections that are needed between the different bottom plates, if several such fuel cell units are connected to a fuel cell device.
Further disadvantages with previously known technology is that the channels which are formed in the covering plate cause a quite high fall-off pressure, which reduces the degree of efficiency of the system and makes the distribution of air and fuel more difficult.
If a system of fuel cell units shall be commercially practicable, it is required that the system have a small volume in relation to the amount of power that is produced. It is also necessary that the system be simple to manufacture and inexpensive to manufacture.